Method and system of illumination



Aug. 29, 1933. R, N. FALGE METHOD AND SYSTEM OF ILLUMINATION Filed Aug. 26, 1929 W m H a w a Patented Aug. 29, 1933 UNITED STATES PATENT OFFICE Robert N. Falge,

Detroit, Mich., assignor to General Motors Research Corporation, Detroit, Mich., a corporation of Delaware Application August 26,

2 Claims.

This invention relates to incandescent lamps and methods of designing and using them so that the life of the bulb may be very greatly increased compared with bulbs of present day design with- 5 out decreasing the illuminating value of the light.

The invention has particularly to do with miniature lamps such as are now used in automobile headlamps, and it is to this type of lamp that the following specification will be primarily directed but it is to be understood that the broad principles of theinvention may be applied to lamps used for other purposes. The drawing shows a bulb designed for use in the practice of my invention.

It is known that the life of a lamp filament depends upon the temperature at which it is operated. It is usual to operate lamps at such filament temperature as will cause them to emit substantially white light, or light as nearly white as is compatible with reasonable durability of the filament. This is not a fixed and exact temperature; but for a given filament material it lies within comparatively narrow limits, and is herein referred to as its usual rated capacity. The temperature of a given filament, in turn depends upon the current supplied to it, and, of course, increased current is obtained by increasing the voltage applied to the lamp. Consequently if the same bulb be run at lower voltage, and consequently lower current, the filament will not attain so high a temperature and its life will be prolonged. However this will reduce the light output and result in introducing a greater percentage of red rays into the light emitted, giving a yellowish cast to the beam. The reduction in the amount of light emitted is, of course, undesirable, and the observer also finds the change in color of the light to be undesirable as candlepower for candlepower, it seems to reduce the visibility of the objects illuminated.

In the above example, the reduction in current density in the wire filament is the really important factor in maintaining the filament at the lower temperature necessary for long life, and, in general, over the range of filament size in lamps of a given class, the temperature of the filament increases as the current density increases.

In the case of automobile headlighting it is not practical to reduce the voltage applied to the lamp as above suggested for the usual automobile is equipped with a substantially fixed voltage source of power. Under ordinary running conditions, this source is the conventional constant voltage generator driven by the engine. While the desired reduction in current density thus can- 1929. Serial No. 388,403

not be satisfactorily attained by changing the applied voltage, it is possible to accomplish this result by properly designing the filament. Merely increasing the cross-section of the filament is not enough for this will result in increased current flow, and consequently increased current density and increased filament temperature. However by increasing both the cross-section and length of the filament it is possible to obtain the desired decrease in current density. In the simplest case the resistance of the filament may be maintained constant by increasing the length of the filament in such an amount as to ofiset the reduction in resistance produced by increase in cross-section of the filament. With the resistance constant. the current will remain constant, but since the cross-section of the filament is increased the current density is diminished and the filament will not operate at as high a temperature as the conventional filament. Hence its life will be longer. However, it will be noted that the power consumed in heating the filament will remain constant, since neither the voltage nor current have been changed. Such an arrangement is undesirable because at the much lower temperature thus obtained the amount of light emitted would be reduced, while at the same time it would possess the undesirable yellowish cast.

A more desirable result may be obtained byincreasing the length and cross-section of the filament in such ratio that the reduction in resistance resulting from increase in cross-section more than offsets the increase in resistance produced by increase in filament length so that there is a net decrease in resistance with resultant increase in current. The relation 01' the factors in design should, in addition, be such that the increase in current is more than oiIset by the increase in cross-section so that a net reduction in current density is obtained. By following out this procedure in laying out the filament design it is possible to obtain, within limits, any desired candlepower, and at the same time lengthen the life of the bulb. However though the filament be designed as just described for operation at fixed voltage, and the length and cross section be increased in such degree as to obtain increased candle power with decreased current density, the problem still remains that the illuminating value of the light is reduced owing to the yellow character of the light resulting from the excess of red rays produced at the resulting reduced operating temperature.

To overcome this difficulty I have incorporated in my bulb means for removing the excess red rays so that the emitted light will have for all practical purposes the same color value as that obtained from conventional bulbs. This may be done in a number of ways but I have preferred to use a bulb made of glass sufiiciently blue in color to absorb the excess red rays and produce a relatively white beam. It is of course essential that the glass be colored as little as is necessary for the colored glass absorbs the rays and reduces the efiiciency of the lamp.

In carrying out my invention I have succeeded in obtaining lamps having substantially the same candlepower and light-beam color characteristics as standard present day commercial lamps, that have a life from two to five times as long.

The bulb may be of any desired size or shape. It may be designed for one filament or more than one. I have illustrated on the drawing a conventional automobile headlamp bulb 2 having two filaments numbered 4 and 6 respectively and have indicated thereon that the glass envelope is tinted with blue to absorb the red rays and produce the same while light now produced by conventional bulbs.

I am giving below the design data on a number of bulbs I have made up involving this inventive idea. For the sake of comparison I have given at the headof the list the design data on the standard 100 hour life bulb now in general use.

Taking the Long Life Bulb No. 2, which is the design I prefer, it will be noted that this bulb has a life of 300 hours compared with 100 hours for the standard bulb. 'This lamp with the blue glass bulb gives the same candlepower as the standard lamp with clear bulb. Disregarding the line loss and the loss in the battery, the power consumption of the long life lamp is 20.72 watts as compared with 17.03 watts for the standard lamp, and, of course, the etliciency of the long life lamp is reduced. the long life bulb requiring .98 watts per candlepower, while the standard bulb takes but .81 watts per candlepower. If we were to assume that the percentage of light absorbed by the clear glass is zero for the standard bulb. then the absorption by the blue Long Life Bulb No. 2 is 5%. However the measured candlepower is that obtained through the glass envelope and in the case of both bulbs is 21 c. p.

I thus obtain very greatly increased life at. the expense of slightly reduced .eiliciency and increased power consumption. However even in the case of the bulb having five times the life of the present day bulb the increase in total power consumption will not exceed 40%, and will be in the neighborhood of from 6 to 8 watts.

This power is derived from the usual generator, and with present automobiles the factor of safety built into the generator is more than ample to take care of the increased load. The increased load on the generator means, of course. increased load on the engine. but the increase in load is negligible as compared with the total power output of modern engines, and the mechanical loads they carry in driving the car.

As previously stated I have used blue glass for the envelope of the bulb, but if preferred I may use green glass or bluish green glass. All of these colors are very close in, wave length and bluish green is the color that is complementary to red so that when light of these colors are added together while light results. Blue and green are so close to bluish green that I refer to them as substantially complementary in color to red. The red rays that give the undesirable yellowish cast to the beam are absorbed by the colored glass, so that the resultant rays are of substantially the same degree of whiteness as is the light emitted from the conventional bulb.

Instead of employing colored glass in the bulb to absorb the red rays I may employ a filtering screen of complementary color around the bulb or I may employ in the lamp a reflector or lens of the same color. This is not as desirable, however, since state regulations prescribe the maximum candlepower of the bulb that may be used in automobile headlighting, and a light absorbing screen. reflector, or lens will reduce the candlepower available for distribution on the road.

I claim:

1. A lighting system comprising a source of electric current of substantially fixed voltage. a circuit. a lamp having a filament designed to operate at such current density and temperature below the usual rated capacity of the filament material as will cause it to emit light having an excess of red rays, and a ray filter adapted to absorb the excess red rays and transmit substantially white light.

2. A method of illumination which comprises operating an electric lamp filament at a current density and temperature below its usual rated capacity to cause it to emit light having an excess of red rays. and filtering out the excess of red rays to cause the resulting light to be substantially white.

ROBERT N. FALGE. 

